This invention is directed to a method for accurate front-to-back alignment of patterns onto a wafer, particularly in the fabrication of surface emitting device such as LED's or lasers. Where fiber optic coupling to the LED is required the most successful scheme for 1.3 um surface emitting LED is the Burrus structure. In this LED structure the high intensity light generated in the epitaxial layers under the top ohmic contact (25-50 microns diameter) is emitted through a hole (125 microns diameter) in the metal contact on the back surface, thus providing a well defined narrow beam with vastly improved fiber coupling efficiency. The substrate is transparent to the emitted light, and the hole in the back surface is accurately aligned to be concentric with the metal contact on the top surface.
Conventionally alignment of a pattern on the bottom surface of a wafer to one on the top surface, i.e. front-to-back alignment, is done with either an infrared mask aligner for aligning through the substrate, or by requiring the wafer to have straight edges and using the straight edges of the wafer as references for both top and bottom patterns.
In applicant's invention an arbitrarily shaped wafer can be accurately aligned without an infrared aligner. In this method specially designed alignment markers which are part of the metallization on the front side of the wafer are exposed by etching away a small area along the edge of the wafer. The backside pattern is then directly aligned to the frontside pattern using these markers.